Turbomachines are operated at a high speed. Accordingly, in order to cool the turbomachines or keep lubricity in the turbomachines, lubricating oil is typically used for lubricating bearings of the turbomachines. When a high-speed bearing is used in an atmosphere of refrigerant as in a refrigeration cycle and a Rankine cycle, an appropriate lubricating oil designed expecting viscosity reduction due to the atmospheric gas is typically used.
In such a refrigeration cycle and Rankine cycle, a lubrication in a fluorine compound refrigerant atmosphere (liquid refrigerant lubrication) is typically applied. Examples of the lubricating oil (base oil) suitable for the liquid refrigerant lubrication are base oils compatible with the refrigerant such as a polyoxyalkylene glycol, ester, polyvinyl ether and alkylbenzene (see, for instance, Patent Literatures 1 and 2).
On the other hand, the use of grease-lubricated bearing by grease under the liquid refrigerant lubrication can simplify a circulation system of the lubricating oil and a separator for separating the refrigerant and the lubricating oil and is advantageous in terms of size and weight reduction and improvement in efficiency. However, when grease-lubricated bearing is used in a refrigerant atmosphere, the grease is washed to be removed (eluted). Accordingly, it has been believed to be extremely difficult to perform grease lubrication in a refrigerant atmosphere. In this connection, Patent Literature 3 discloses a use of grease containing MoS2 in a liquid refrigerant lubrication. However, the composition of the main component of the grease is unknown.